Pain is one of the most important and feared symptoms of disease. Pain can be broadly divided into three classes:
1. Pain with a clearly defined cause which activates a normal nervous system and which can be effectively treated by removal of the cause, and usually alleviated by analgesics such as non-steroidal anti-inflammatories and opiates. Examples are pain due to trauma, infection or pathology such as an invading cancer.
2. Pain which appears to originate in damage to the central or peripheral nervous system itself and which may persist long after the original cause of the damage has been removed. This type of pain is usually called neuropathic or neuralgic and has many causes. Any form of trauma or other damage to any peripheral nerve or to certain parts of the central nervous system may be followed by prolonged pain which may persist for months, years or decades. The damage may be caused by accidental or surgical injury, by metabolic disturbances such as diabetes or vitamin B12 or other nutrient deficiency, by ischaemia, by radiation, by autoimmune attack, by alcohol, by infections, especially viral infections, particularly with the herpes virus, by tumours, by degenerative diseases, or by unknown factors such as may be operative in trigeminal and other neuralgias. This is by no means an exhaustive list. These types of pain often respond poorly to treatment and patients suffering them have frequently been subjected to trials of many different drugs without success. The most consistent successes are perhaps achieved with antidepressant drugs of various types. A drug for temporal lobe epilepsy, carbamazepine, is sometimes effective in trigeminal neuralgia though not usually in other types of pain in this class.
3. Pain of indeterminate origin. Many pains cannot easily be classified into one or other of these types. It is not clear whether many headaches and migraines are in type 1 or type 2. Low back pain is also often difficult to define.
Any individual who is experiencing pain immediately wants it relieved. Fortunately pain of the first type can often be treated successfully although this is not always the case. Pain of the second and third types is often poorly responsive to existing methods of treatment and many patients have tried large numbers of drugs without real success. The problems of dealing with such pain have been well described in recent articles such as those by J. W. Scadding, pp 3936 to 3946 in the 3rd edition of the Oxford Textbook of Medicine, Oxford University Press, 1996; R. G. Kost and S. E. Straus in the New England Journal of Medicine 335: 32-42, 1996; and B. S. Galer in Neurology, 45, Supplement 9, S17-S25, 1996.